Wading Into Titan’s Mysterious Lakes

A radar image of Titan's Ligeia Mare Image: NASA.

Titan is one of the solar system’s most intriguing and enigmatic objects. Physicists grapple with the challenges of characterizing novel materials 800 million miles away, planetary scientists ponder how terrestrial processes may be expressed under a different physical regime, and astrobiologists debate the prospect of life in liquid hydrocarbons.

In a presentation at the California Institute of Technology earlier this week, Cornell University Professor Alex Hayes revealed some of the latest findings from the Cassini Mission. The spacecraft has been orbiting Saturn and its moons for nearly 10 years, but the last several months have been particularly eventful: The team has been both lucky and good, opening up new methods of analysis that should yield dividends for years to come.

Hayes works primarily with the spacecraft’s radar instrument, whose long wavelength pulses allow the team to “see” through visibly opaque layers like clouds and collect information on a surface’s roughness or moisture content. But painting with such a broad brush does have some drawbacks, and other instruments like Cassini’s infrared spectrometer offer complementary data.

Radar has proven extremely valuable in mapping Titan’s lake networks, which show up as inscrutable pools of darkness in the gold-tinged images beamed back to Earth. The lakes are almost entirely found in the northern hemisphere – the imbalance is possibly due to an asymmetric orbit that could have prompted evaporation in the South – and the larger ones are “comparable in size to the Great Lakes in the Midwest,” according to Hayes.

In 2008, radio waves shot from Cassini toward the surface of a lake named Ontario Lacus reflected right back, like a laser off of a mirror, saturating the instrument’s detector. The blinding flash indicated an almost perfectly flat surface, with less than 3 millimeters of vertical variation across a 100-m wide area. (The full lake is 235 kilometers long.) With a sample size of one, it was impossible to know if Ontario Lacus was representative of all of Titan’s lakes, or if instrumental, physical, and environmental forces may have conspired to provide a unique result.

And so, the team’s observation of Ligeia Mare, a Lake Superior-sized feature near the moon’s north pole, came with great anticipation. Once again, radio waves reflected right back at the spacecraft; when the math settled, the lake’s maximum roughness was determined to be roughly one millimeter.

This remarkable smoothness is unexpected since “it should be easier to make waves on Titan than on Earth,” says Hayes, “and you’d never see this kind of smoothness here.” Liquids on Titan’s surface feel less severe gravitational forces, the surface tension of a methane / ethane mixture is less than that of water, and the atmosphere is thicker, allowing winds to pack an extra punch. In short, the forces that create waves are amplified on Titan, and the factors that resist them are smaller. The solution may lie in seasons of doldrums, when winds stay below 0.4 meters per second, but future observations are in the works.

The glassy lakes of Titan contain other secrets as well, none more stupefying than the case of the disappearing island. The team has a series of grainy radar images, all taken of the same patch of lakeshore, taken over several years. A 2007 image shows a U-shaped shoreline ringing a pitch-black expanse of liquid hydrocarbons. But in July 2013, a radar-bright patch appears, showing the reflective properties of solid ground. Following up on this odd discovery a few months later, the team was even more surprised to process its data set from October 2013, only to find the same tranquil, island-free lake from six years earlier.

Hayes and the rest of the radar team aren’t sure what to make of their data. “It could be a submerged seamount or a transient effect, or an observational artifact,” he explains, leaving all possibilities open. “But we can say that this magic island, if real, behaves unlike anything else we’ve seen on Titan so far.”

This continual evolution of a long-lived, extraordinarily successful mission keeps Hayes excited about Cassini’s future prospects. “The fact that we’re still seeing fundamentally new things after so many years is amazing,” he says, “and who knows what we’ll find next?”